New Solid Solution and Phase Equilibria in the Subsolidus Area of the Three-Component CuO–V2O5–Ta2O5 Oxide System

The results of the study of the three-component system of CuO–V2O5–Ta2O5 oxides showed, inter alia, that in the air atmosphere in one of its cross-sections, i.e., in the CuV2O6–CuTa2O6 system, a new substitutional solid solution with the general formula CuTa2−xVxO6 and homogeneity range for x > 0.0 and x ≤ 0.3 is formed. The influence of the degree of incorporation of V5+ ions into the CuTa2O6 crystal lattice in place of Ta5+ ions on the unit cell volume, thermal stability and IR spectra of the obtained solid solution was determined. Moreover, the value of the band gap energy of the CuTa2−xVxO6 solid solution was estimated in the range of 0.0 < x ≤ 0.3, and on this basis, the new solid solution was classified as a semiconductor. On the basis of the research results, the studied system of CuO–V2O5–Ta2O5 oxides was also divided into 12 subsidiary subsystems.


Introduction
The three-component system of metal oxides CuO-V 2 O 5 -Ta 2 O 5 has not been studied so far in terms of the phases formed in it, and also in order to determine the phase equilibria established in this system over the entire range of concentrations of components in the air atmosphere in the solid state. In turn, the CuO, V 2 O 5 and Ta 2 O 5 oxides, which are the components of the system selected for the study, belong to an interesting class of compounds, both in terms of their crystal structure and physicochemical properties. These oxides, due to their electrical and optical properties, are commercially important materials that are used in optoelectronic devices, energy conversion and as catalysts or photocatalysts [1][2][3][4][5][6][7][8][9][10][11][12]. Copper (II) oxide is used to obtain optical switches, field emission devices, lithium-ion electrode materials, gas sensors, biosensors and magnetic data carriers [1][2][3][4]. The applications of vanadium (V) oxide include catalytic processes and the production of field-effect transistors, gas sensors, infrared detectors, glass dyes [5][6][7][8][9][10]. Tantalum (V) oxide, due to its high dielectric constant, is used in the production of semiconductors used in DRAM memories, high-frequency CMOS integrated circuits and flash memories [11,12].
It is known from the available literature that the CuTa 2 O 6 compound is formed in the binary system of CuO-Ta 2 O 5 oxides [38][39][40][41]. Three crystallographic forms of CuTa 2 O 6 are known: monoclinic, cubic and tetragonal [38][39][40]. Vincent and his colleagues report that there is also an orthorhombic form of the CuTa 2 O 6 compound [41]. The temperature of transition of polymorphic monoclinic to tetragonal variety is 227 • C [38]. Due to its properties, the CuTa 2 O 6 compound improves the piezoelectric properties of ceramics used in the production of transformers, transducers, ultrasonic motors and devices for surface acoustic waves [42][43][44].
The large variety of properties of the oxides that build the CuO-V 2 O 5 -Ta 2 O 5 ternary system, as well as the compounds formed in its side systems, such as CuTa 2 O 6 and CuV 2 O 6 , make them interesting for technical applications. Therefore, it seemed justified to study the mutual reactivity of both the oxides and the known compounds in order to find out whether they react with the formation of new phases with potential, interesting application properties.
The presented work also includes the results of research on phase equilibria established in the title three-component system of metal oxides in the entire range of concentrations of the components of this system. The results of these studies allowed us to determine, inter alia, the ranges of concentrations of the components of the tested system and the temperatures in which the identified phases (both known and obtained for the first time in this work) coexist in the solid state.

Materials and Methods
The following oxides were used for the tests: CuO p.a. (Aldrich, St. Louis, MO, USA), V 2 O 5 p.a. (POCh, Gliwice, Poland) and Ta 2 O 5 p.a. (Aldrich, St. Louis, MO, USA). The reactants were weighed in the appropriate amounts, homogenized by trituration, pelleted and heated in an air atmosphere under conditions that allowed the reaction to proceed in the solid phase. The samples were heated in the temperature range of 550-900 • C in several 24-h stages.
After each heating stage, the samples were left in the furnace until they cooled to room temperature and then were ground and tested by XRD to determine their phase composition.
The temperatures of the final stage of heating the samples and the kind of phases in equilibria are given in Tables 1 and 2.
The powder diffractograms of the tested phases were recorded at room temperature using an Empyrean II diffractometer (CuKα radiation, graphite monochromator, semiconductor PIXcel3D detector, measuring step 0.013 • , counting time in the measuring interval 70s, manufacturer PanAnalytical, The Netherlands). The phases were identified using the data from the ICDD 2021 PDF database. The Refinement program (DHN, Poland) was used to indexation the powder diffraction patterns.  DTA-TG studies were performed with the F. Paulik-L. Paulik-L. Erdey derivatograph, MOM Budapest, Hungary. The measurements were made in the air atmosphere, in the temperature range 20-1000 • C, with the galvanometer's sensitivity of DTA 1/5 and the constant heating rate of 10 • C/min. All tests were performed in quartz crucibles. The mass of the tested samples was always 500 mg. The accuracy of the temperature reading, determined on the basis of iterations, was found to be ±5 • C.
Selected samples were tested by the DTA-TG method using the SDT 2960 from TA Instruments. Measurements were carried out in an air atmosphere, in the temperature range of 20-1500 • C, at a heating rate of 10 • C/min. Measurements were made in corundum crucibles. The mass of the tested samples was~20 mg.
The IR spectra were recorded with the help of the SPECORD M 80 IR spectrometer, manufactured by Carl Zeiss, Jena, East Germany, using the KBr pellet pressing technique in a mass ratio of 1:300. Measurements were made in the range of wavenumber 1200-200 cm −1 .
A UV-Vis-V-670 spectrophotometer (JASCO, Tokyo, Japan) with an integrating sphere PIV-756/PIN-757 was used for the UV-Vis spectroscopy tests. The optical absorption was measured in the 200-1000 nm range at room temperature.
As part of the presented work, tests of phase equilibria in the CuO-V 2 O 5 -Ta 2 O 5 system were additionally carried out in the air atmosphere in a solid state. In this stage of the study, 20 samples made of oxides were synthesized in conditions ensuring achievement of the equilibrium state, and the achievement of such a state was found when the results of the samples, carried out with the XRD and DTA methods after two successive heating stages, were identical, and the number of phases was consistent with the extended Gibbs phase rule.

Reactivity of CuO and V 2 O 5 with Ta 2 O 5 in the Air
The study of the three-component system of CuO-Ta 2 O 5 -V 2 O 5 oxides began with the examination of one of the hypothetical cross-sections of this system, i.e., CuV 2 O 6 -CuTa 2 O 6 . For this purpose, ten samples were prepared from the separately obtained compounds, i.e., CuTa 2 O 6 and CuV 2 O 6 , with compositions selected to represent the entire range of concentrations of the components of the tested system ( Table 1). The location of weighed samples compositions is marked in the area of the triangle of concentrations of the components of the CuO-Ta 2 O 5 -V 2 O 5 system.
The synthesis of the compound CuTa 2 O 6 was carried out in an air atmosphere according to the reaction equation below: Under the following conditions: 900 The obtained compound showed a green color, and the position of the lines in the diffraction pattern was consistent with the data given in the PDF card 00-032-0349, which proved that the tetragonal form of the CuTa 2 O 6 compound was obtained.
In order to obtain copper (II) metavanadate (V), the sample consisting of oxides with the composition: 50 mol% CuO and 50 mol% V 2 O 5 was weighed and subjected to two-stage 24-h heating at the following temperatures: 550 • C and 600 • C. A plum gray compound was obtained. X-ray phase analysis of the sample after the last heating step confirmed that the diffraction lines characterizing this compound are consistent with the data contained in the PDF card of the triclinic modification polymorph of the CuV 2 O 6 compound with the number 00-045-1054, i.e., α-CuV 2 O 6 .
The results of the XRD analysis of the tested samples after the last heating stage and the temperature of the last heating stage are presented in Table 1.
The phase compositions of samples No. 1 and No. 2 in the equilibrium state presented in Table 1 and contained in the initial mixtures were 90.00 mol% CuV 2 O 6 and 10.00 mol% CuTa 2 O 6 , and 75.00 mol% CuTa 2 O 6 and 25.00 mol% CuV 2 O 6 , respectively, indicating that the samples reacted as: 3 CuV 2 O 6(s) + CuTa 2 O 6(s) = 2 Cu 2 V 2 O 7(s) + 2 TaVO 5(s) (2) In sample 1, the components CuV 2 O 6 and CuTa 2 O 6 in the reaction mixture remained in a molar ratio of 9:1; therefore, with respect to the stoichiometry of the reaction taking place in the reaction mixture, the CuV 2 O 6 substrate is in excess and therefore, this compound is ultimately in equilibrium with the reaction products (2).
The phase composition of samples 3 and 4 at equilibrium shows that the CuV 2 O 6 present in the samples reacted with CuTa 2 O 6 with the formation of three compounds Cu 2 V 2 O 7 , TaVO 5 and Ta 9 VO 25 . Thus, in sample No. 3, the initial mixture of which contained 66.67 mol% of CuV 2 O 6 and 33.33 mol% CuTa 2 O 6 , the reaction proceeded according to the following reaction equation: 16 CuV 2 O 6(s) + 8 CuTa 2 O 6(s) = 12 Cu 2 V 2 O 7(s) + 7 TaVO 5(s) + Ta 9 VO 25(s) The presented results of the X-ray analysis of samples 5-9 after the last heating stage show that in these samples, the reaction between CuV 2 O 6 and CuTa 2 O 6 resulted in the formation of compounds, i.e., Cu 2 V 2 O 7 and Ta 9 VO 25 . The excess substrate in these reactions is CuTa 2 O 6 . For sample No. 7, which in the initial mixture contained 40.00% mol of CuV 2 O 6 and 60.00% mol of CuTa 2 O 6 , the reaction proceeded according to the reaction equation: 11 CuV 2 O 6(s) + 9 CuTa 2 O 6(s) = 10 Cu 2 V 2 O 7(s) + 2 Ta 9 VO 25(s) (4) Heating of sample 10 at the temperature of 900 • C resulted in the presence of only lines characterizing the compound CuTa 2 O 6 in the diffraction pattern. These lines were slightly shifted towards the higher angles 2θ, i.e., they corresponded to smaller values of the d hkl interplanar distances. It was found that the shift of the diffraction lines belonging to the CuTa 2 O 6 set and the absence of the Cu 2 V 2 O 7 compound in the sample proves that a substitutional solid solution is formed in the sample, the matrix of which is CuTa 2 O 6 , according to the reaction equation: In order to determine the homogeneity range of the formed solid solution, additional samples were prepared from the finished phases, i.e., CuTa 2 O 6 and CuV 2 O 6 , containing 12.5 mol%, 15.00 mol% and 17.5 mol% CuV 2 O 6 , respectively, in the initial mixtures. Xray phase analysis showed that only the samples with the initial composition of 12.5 mol% CuV 2 O 6 and 87. 50  The chemical formula of the formed solid solution was determined on the basis of the composition of the initial mixture of substrates, taking into account their complete conversion, which was found on the basis of the X-ray phase analysis of the synthesized samples after the last stage of heating them at 900 • C ( Table 2).
In Table 2, apart from the composition of the initial mixtures of substrates from which single-phase samples containing a solid solution were obtained, the phase composition of the sample after exceeding its homogeneity is given.
At this stage of the research, however, it cannot be ruled out that in the obtained single-phase samples containing the solid solution, no other phases are still present, but if so, they are in amounts not detectable by the XRD method. However, this does not affect the general formula of the obtained new solid solution and the proposed range of its homogeneity.
Based on the research, it was found that the CuTa 2−x V x O 6(s.s.) solid solution formed in the three-component CuO-V 2 O 5 -Ta 2 O 5 oxide system has a homogeneity range of 0 < x ≤ 0.3, which shows that the maximum degree of incorporation of V 5+ ions in place of ions Ta 5+ in the CuTa 2 O 6 crystal lattice is 15 mol%.
As part of the work, samples containing a CuTa 2−x V x O 6 solid solution, where x = 0.30 in equilibrium with other phases, were annealed at higher temperatures and then frozen (rapidly cooled to room temperature) in order to determine the change in their phase composition. XRD analysis did not show such a change, which means that the solubility range of the synthesized solution does not significantly depend on the temperature. with an increase in vanadium content in CuTa2−xVxO6 samples. The analysis of t sented diffractograms showed that with the increase of the degree of incorporation nadium ions into the crystal lattice of the CuTa2O6 matrix, the diffraction refl shifted towards higher 2θ angles up to x = 0.30. Indexing of the powder diffraction patterns of the CuTa2−xVxO6 solid solution 0.20; 0.25 and 0.30 and the CuTa2O6 compound were made using the REFINEMEN gram. The results of the indexing, presented in Table 3, indicate that with the incr the degree of incorporation of V 5+ ions in place of Ta 5+ ions into the CuTa2O6 crystal the a = b and c parameters of unit cells and the volume of unit cells decrease, a crystal lattice becomes contracted. The   Table 3, indicate that with the increase of the degree of incorporation of V 5+ ions in place of Ta 5+ ions into the CuTa 2 O 6 crystal lattice, the a = b and c parameters of unit cells and the volume of unit cells decrease, and the crystal lattice becomes contracted. The thermal stability of the CuTa 2−x V x O 6 solid solution was determined by performing DTA-TG tests of all single-phase samples for x = 0.00, 0.20, 0.25 and 0.30. Meanwhile, in the DTA curves of all samples, made up to 1500 • C, one endothermic effect was recorded, with an onset temperature in the range of 1270-1350 • C, depending on the composition of the sample. The temperature of the onset of the effects recorded for all samples was always lower than the melting point of CuTa 2 O 6 , and it decreased with the increasing degree of incorporation of V 5+ ions. DTA curves of the tested samples are not included in this work due to the recorded very small thermal effect.
In order to characterize the properties more closely and confirm the structure of the obtained CuTa 2−x V x O 6 solid solution, IR spectra of all its single-phase samples (x = 0.2, 0.25 and 0.3) and the CuTa 2 O 6 matrix were recorded. Figure 2 shows the IR spectra of CuV 2 O 6 (curve a), CuTa 2−x V x O 6 solid solution for x = 0.3, with the highest degree of incorporation of V 5+ ions into the CuTa 2 O 6 crystal lattice (curve b) and the CuTa 2 O 6 compound (curve c). High distortion of MO x polyhedra characteristic for compounds of vanadium, tantalum and copper makes analysis of their IR spectra very difficult [45][46][47][48][49].
Materials 2022, 15, x FOR PEER REVIEW In order to characterize the properties more closely and confirm the structur obtained CuTa2−xVxO6 solid solution, IR spectra of all its single-phase samples (x = and 0.3) and the CuTa2O6 matrix were recorded. Figure 2 shows the IR spectra of (curve a), CuTa2−xVxO6 solid solution for x = 0.3, with the highest degree of incorp of V 5+ ions into the CuTa2O6 crystal lattice (curve b) and the CuTa2O6 compound (c High distortion of MOx polyhedra characteristic for compounds of vanadium, ta and copper makes analysis of their IR spectra very difficult [45][46][47][48][49]. The IR spectrum of CuV2O6 comprised two broad absorption bands with ma 864 and 550 cm -1 (Figure 2, curve a). The absorption band with a maximum at 864 c be ascribed to stretching vibrations of V-O bonds in distorted VO6 octahedra (char tic for CuV2O6 structure) [48,50]. IR spectra of solid solution CuTa1.70V0.30O6 (x = 0.3 ure 2, curve b), and its matrix CuTa2O6 (Figure 2, curve c) contains four band boundaries of 950-850 cm -1 , 750-550 cm -1 , 520-430 cm -1 and 380-300 cm -1 with ma about 890, 640, 460 and 330 cm -1 . It follows from the literature survey that these ba characteristic of solid tantalates (V) and vanadates (V) [48,[50][51][52][53]. The similarity IR spectra corroborates that the solid solution adopts the CuTa2O6 structure. An in band with an absorption maximum registered in the region 950-850 cm -1 at ~870 c be attributed to the stretching vibrations of Ta-O and V-O bonds in distorted T TaO7 as well as VO4 and VO6 polyhedra [48,49,51,52,54]. Bands in the range of 75 cm -1 and 520-430 cm -1 can be attributed to the stretching vibrations of the Cu-O b the distorted CuOx polyhedra [45,46] [48][49][50]53].
The comparative analysis of positions and intensities of absorption bands in IR spectra of CuTa 2 O 6 and CuTa 2−x V x O 6 solid solution samples has revealed that incorporation of vanadium ions in the crystal lattice of CuTa 2 O 6 mainly affects the position of the band with a maximum at 600 cm −1 , which shifts gradually towards higher wavenumbers with an increase of vanadium content reaching 640 cm −1 in the spectrum of CuTa 2−x V x O 6 for x = 0.3 (Figure 2, curve d). The affection of the position of only one absorption band as a result of the increase of vanadium content seems to indicate that the crystal structure of the solid solution is built up of at least two crystallographically-independent polyhedra, and this less distorted one is preferentially occupied by vanadium ions.
In the next stage of works, in order to qualify the new phase obtained by the class of conductors, insulators or semiconductors, single-phase samples containing the CuTa 2−x V x O 6 solid solution were tested by the UV-Vis-DRS method. The UV-vis diffuse reflectance spectra were converted to absorbance spectra by the Kubelka-Munk method [55]. Figure 3 shows the Uv-Vis absorption spectra of compounds: CuTa 2 O 6 (curve a), CuV 2 O 6 (curve b) and a solid solution of CuTa 2−x V x O 6 for x = 0.30 (curve c). aterials 2022, 15, x FOR PEER REVIEW 8 the increase of vanadium content seems to indicate that the crystal structure of the s solution is built up of at least two crystallographically-independent polyhedra, and less distorted one is preferentially occupied by vanadium ions.
In the next stage of works, in order to qualify the new phase obtained by the cla conductors, insulators or semiconductors, single-phase samples containing CuTa2−xVxO6 solid solution were tested by the UV-Vis-DRS method. The UV-vis dif reflectance spectra were converted to absorbance spectra by the Kubelka-Munk me [55]. Figure 3 shows the Uv-Vis absorption spectra of compounds: CuTa2O6 (curv CuV2O6 (curve b) and a solid solution of CuTa2−xVxO6 for x = 0.30 (curve c). In addition, the values of the band gap energy were estimated for the CuTa2O6 CuV2O6 compounds and the solid solution formed as a function of the degree of inco ration (x) of V 5+ ions into the CuTa2O6 crystal lattice in place of Ta 5+ ions. It was found the value of the energy gap for CuV2O6 is 1.85 eV, and the energy gap for the CuTa2−xV solid solution ranges from 2.75 to 2.47 eV for 0.0 ≤ x ≤ 0.30 (Figure 4). These results s that the magnitude of the energy gap decreases with increasing the degree of incorp tion of V 5+ ions in the CuTa2O6 crystal lattice in place of Ta 5+ ions. In addition, the values of the band gap energy were estimated for the CuTa 2 O 6 and CuV 2 O 6 compounds and the solid solution formed as a function of the degree of incorporation (x) of V 5+ ions into the CuTa 2 O 6 crystal lattice in place of Ta 5+ ions. It was found that the value of the energy gap for CuV 2 O 6 is 1.85 eV, and the energy gap for the CuTa 2−x V x O 6 solid solution ranges from 2.75 to 2.47 eV for 0.0 ≤ x ≤ 0.30 (Figure 4). These results show that the magnitude of the energy gap decreases with increasing the degree of incorporation of V 5+ ions in the CuTa 2 O 6 crystal lattice in place of Ta 5+ ions.
The determined values of the band gap energy indicate that the new solid solution is a semiconductor.

Phase Equilibria in the Subsolidus Region of the CuO-Ta 2 O 5 -V 2 O 5 System
The phase composition of the tested samples (1-10) after the last heating stage, i.e., in the state of equilibrium (Table 1)  The determined values of the band gap energy indicate that the new solid solution is a semiconductor.

Phase Equilibria in the Subsolidus Region of the CuO-Ta2O5-V2O5 System
The phase composition of the tested samples (1-10) after the last heating stage, i.e., in the state of equilibrium (Table 1)    The determined values of the band gap energy indicate that the new solid solution is a semiconductor.

Phase Equilibria in the Subsolidus Region of the CuO-Ta2O5-V2O5 System
The phase composition of the tested samples (1-10) after the last heating stage, i.e., in the state of equilibrium (Table 1)   The final verification of the subsolidus area of the CuO-Ta 2 O 5 -V 2 O 5 system consisted in the preparation of additional samples (Nos. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] constituting mixtures of these phases, which, based on the results of previous studies, were considered to be equilibrium and represent real two-component systems (samples containing two phases in a state of equilibrium) or partial systems (samples containing three phases in equilibrium). The compositions of these mixtures, as calculated for the components of the CuO-Ta 2 O 5 -V 2 O 5 system, corresponding to the compositions of the samples, are presented in Table 4. The prepared mixtures of phases were subjected to long-term heating at temperatures slightly lower (by~20 • ) than the temperatures of the first effects recorded on their DTA curves. XRD analysis of these preparations showed that despite many hours of heating at temperatures close to the onset of melting, the phase composition of the samples did not change. This proves that the initial mixtures corresponded with their composition to the predetermined coexisting phases in the equilibrium state in individual systems.
The data presented in Table 3 not only confirmed the conclusions resulting from previous studies on the CuV 2 O 6 -CuTa 2 O 6 binary system, but also allowed us to divide the CuO-Ta 2 O 5 -V 2 O 5 system into twelve partial systems ( Figure 5), i.e., On the basis of the DTA tests of samples after the last heating stage, the temperatures to which the phases representing individual partial systems and the relevant real twocomponent systems coexist in the solid state were determined. The temperatures of the onset of the first endothermic effect, recorded on the DTA curves of preparations in the state of equilibrium, and corresponding to a given partial system or a real two-component system, were assumed as these temperatures. The melting points of the mixtures of the phases coexisting in the given system are given in Table 5. The DTA curves of selected samples recorded in the air atmosphere up to 1000 • C are shown in Figure 6.
The presented DTA curves of selected samples in the equilibrium state showed, among others, that the partial V 2 O 5 -TaVO 5 -CuV 2 O 6 system (Figure 6a) melts eutectically at a temperature of 625 ± 5 • C, which means that its components coexist in the solid state to this temperature. Determining the composition of the triple eutectic mixture formed in this constituent system is beyond the scope of this work and will be determined in future studies.
In this part of the study, it was also established that the components of the second partial system CuV 2 O 6 -TaVO 5 -Cu 2 V 2 O 7 coexist in the subsolidus area up to the temperature of 640 ± 5 • C (Figure 6b). This temperature is related to the smelting of the eutectic mixture formed between CuV 2 O 6 and Cu 2 V 2 O 7 in the two-component V 2 O 5 -CuO system [13][14][15][16][17][18][19][20][21]. The presented DTA curves of selected samples in the equilibrium state showed, among others, that the partial V2O5-TaVO5-CuV2O6 system (Figure 6a) melts eutectically at a temperature of 625 ± 5 °C , which means that its components coexist in the solid state to this temperature. Determining the composition of the triple eutectic mixture formed in this constituent system is beyond the scope of this work and will be determined in future studies.
In this part of the study, it was also established that the components of the second partial system CuV2O6-TaVO5-Cu2V2O7 coexist in the subsolidus area up to the temperature of 640 ± 5 °C (Figure 6b). This temperature is related to the smelting of the eutectic Based on the DTA curve of a sample containing TaVO 5 at equilibrium with Cu 2 V 2 O 7 and with a saturated solid solution of Ta 9−x V x O 25 (Figure 6c), it was found that these phases coexist in the solid state up to 730 ± 5 • C. In this case, it is related to the melting of the TaVO 5 eutectic mixture with Cu 2 V 2 O 7 .
Moreover, it was found that the solid solution obtained as part of this work, with the structure of CuTa 2 O 6 and the maximum degree of incorporation of V 5+ ions, remains at equilibrium in the solid state with Ta 9−x V x O 25 and Cu 2 V 2 O 7 , as well as Cu 2 V 2 O 7 and Cu 3 V 2 O 8 up to 760 and 740 ± 5 • C, respectively (Figure 6d). The melting point of subsystem IV is related to the congruent melting of Cu 2 V 2 O 7 , and the VIII subsystem to the melting of the eutectic mixture formed between Cu 2 V 2 O 7 and Cu 3 V 2 O 8 [15,21].
It was additionally established that CuTa 1.7 V 0.3 O 6 coexists in the subsolidus region with Cu 3 V 2 O 8 and Cu 11 V 6 O 26 to~780 • C ( Figure 6e) and with Cu 11 V 6 O 26 and Cu 5 V 2 O 10 to~770 • C, while with Cu 5 V 2 O 10 and CuO up to 820 • C (Figure 6f). These results indicate that the fields IX and XI of the tested ternary oxide system V 2 O 5 -CuO-Ta 2 O 5 ( Figure 5) melt peritectically but X eutectically. It is related to both the incongruent melting of Cu 3 V 2 O 8 and Cu 5 V 2 O 10 compounds and the eutectic formed between Cu 11 V 6 O 26 and Cu 5 V 2 O 10 [15,20,26].
The exact melting points of the partial systems V-VII and XII were not determined because no thermal effects were recorded on the DTA curves of samples in equilibrium state, representing these fields, in the temperature range up to 1000 • C.
Nevertheless, on the basis of such observation, it can be stated without doubt that the mixtures of components of the mentioned subsystems melt above 1000 • C.
The results of these studies are of great practical importance, including when designing new multi-component materials, such as, e.g., catalysts. They provide important information on the type of phases formed in the CuO-V 2 O 5 -Ta 2 O 5 system, and at what temperatures these phases coexist with each other in the solid state, i.e., their mixtures can be used without the risk of melting and the associated change in their composition.

Conclusions
The research results obtained as part of this work authorize the following conclusions: 1.
In the three-component system of CuO-V 2 O 5 -Ta 2 O 5 oxides, a substitution solid solution is formed with a limited range of homogeneity and the general formula CuTa 2−x V x O 6 for 0 < x ≤ 0.3.

2.
The new solid solution is formed by the incorporation of V 5+ ions in the CuTa 2 O 6 crystal lattice in place of Ta 5+ ions. The maximum degree of V 5+ ion incorporation is 15 mol%.

3.
CuTa 2−x V x O 6 for 0 < x ≤ 0.3 crystallizes in the tetragonal system, and with the increase of the degree of incorporation of V 5+ ions in place of Ta 5+ ions into the CuTa 2 O 6 crystal lattice, the parameters a = b and c and the volume of unit cells decrease, and the crystal lattice contracts. 4.
The CuTa 2−x V x O 6 solid solution is stable, depending on its composition, from a temperature of 1350 • C for x = 0.00 to 1270 • C for x = 0.30.

5.
The IR spectra of solid solution CuTa 2−x V x O 6 (0 < x ≤ 0.30) and its matrix CuTa 2 O 6 are very similar what corroborates their isostructural character. IR spectra of these phases contain bands in the boundaries of 950-850 cm −1 , indicating that crystal lattices of these phases are built up of considerably distorted polyhedra. The incorporation of vanadium ions in the crystal lattice of CuTa 2 O 6 mainly affects the position of the band with a maximum at 600 cm −1 , which shifts gradually reaching 640 cm −1 in the spectrum of CuTa 2−x V x O 6 for x = 0.30. 6.
The CuTa 2−x V x O 6 solid solution is a semiconductor, and the value of the energy gap for the solid solution ranges from 2.75 to 2.47 eV for 0.00 ≤ x ≤ 0.30. 7.
The three-component system of metal oxides CuO-V 2 O 5 - Ta